1. Field of the Invention
This invention relates to a method of making a nitride semiconductor substrate composed of InXAlYGa1−X−YN: 0≦X≦1, 0≦Y≦1, and 0≦X+Y≦1, and a nitride semiconductor substrate made by the method.
2. Description of the Related Art
Nitride semiconductor crystals are very difficult to grow in bulk form from a melt by boat method or Czochralski method since nitrogen has a very high vapor pressure. A very small substrate can be only obtained by these methods to use the principle where desorption of group V element is suppressed by applying high pressure. For this reason, a nitride semiconductor substrate, so-called “free-standing substrate” is produced such that a nitride semiconductor layer is grown mainly by vapor growth method on a hetero-substrate such as a sapphire substrate, a silicon substrate and a gallium arsenide substrate which is made of a material different from the nitride semiconductor, and the hetero-substrate is then removed by grinding, etching or peeling etc. so that only the grown nitride semiconductor layer remains.
For example, methods for producing the free-standing substrate are disclosed in JP-A-2002-57119, JP-B-3631724, JP-B-3744155 and JP-B-3788041.
Some methods are conducted such that only one free-standing substrate is taken from the remaining nitride semiconductor layer after removing the hetero-substrate. However, JP-A-2002-29897 discloses a method that a thick nitride semiconductor layer is epitaxially grown and is then sliced to yield plural free-standing substrates.
Herein, “free-standing substrate” means all free-standing substrates as produced by the above methods.
Vapor growth methods as mentioned earlier are generally used to grow the free-standing substrate. Of them, HVPE (hydride vapor phase epitaxy) is in general deemed to be fast in crystal growth rate and most suitable in industrial production. The details of HVPE are described in JP-A-2002-57119, JP-B-3631724, JP-B-3744155, JP-B-3788041 and JP-A-2002-29897. Of the other methods, MOVPE (metalorganic vapor phase epitaxy) is not suited to produce a thick film such as the free-standing substrate since its growth rate cannot be increased so much. The other methods include the known sublimation method and liquid phase growth method, which are not generally available to grow the free-standing substrate.
Where a nitride semiconductor layer is grown on a hetero-substrate such as sapphire having a lattice constant significantly different from that of nitride semiconductor, it cannot be epitaxially grown while coherently inheriting the lattice structure of the hetero-substrate. Therefore, the crystal growth process is developed such that nuclei for the crystal growth are first generated at some positions on the hetero-substrate, crystals are then gradually grown originating at the nuclei, and the adjacent crystals are then connected to form a planarly continuous film with a flat surface. In order to lower the dislocation density of the crystal obtained by the above process, it is desired to lower the density of nuclei generated initially. However, if the density of nuclei generated lowers, the connection frequency of adjacent crystals is reduced so that it becomes difficult to obtain the planarly continuous film with a uniform thickness. This may cause a relatively steep recess in the direction from the top surface to the back surface of the film.
Where the free-standing substrate obtained by removing the hetero-substrate is ground to have a desired thickness, the grinding is generally conducted simultaneously at a surface (hereinafter called “top surface” of the substrate) on the growth side of the nitride semiconductor as well as at a surface (hereinafter called “back surface” of the substrate) on the side of the hetero-substrate removed. If the degree of grinding at the back surface is high, there may be formed a penetrating pit which penetrates from the top surface to the back surface of the substrate. Further, where the steep recesses as described above are linearly connected together like a crack, there may be formed a penetrating crack which penetrates from the top surface to the back surface of the substrate. Even if only one penetrating pit or penetrating crack is formed in the substrate, the substrate cannot be held by vacuum suction during the device fabrication process such as epitaxially growing a semiconductor layer on the substrate. In other words, it is useless to transfer the defective substrate to the device fabrication process.
Thus, since the substrate having such a penetrating pit or penetrating crack is substantially useless in the device fabrication process even when being transferred thereto, the production yield of the substrate will be significantly reduced.